1. Field of the Invention
This invention relates to an arrangement for enhancing thermal conduction to facilitate the removal of heat from a semiconductor chip implemented module and, more particularly, to an arrangement for accomplishing such cooling in a module which employs an interfacial layer of liquid metal alloy coated metallic dendrites.
2. Description of the Prior Art
One of the principal difficulties in the quest toward greater circuit density and higher performance in the large scale integration of bipolar circuits has proven to be the removal of heat from the chips which is generated by the dissipation of electrical energy. Increasing the power dissipation by increasing the circuitry per unit of chip area with no other change implies a proportionate increase in chip operating temperature. This would, unfortunately, result in a reduction of device life according to an exponential, rather than a proportional relationship.
As a consequence, much effort has been devoted to improvement of packaging techniques so that the thermal resistance between a chip and its ambient environment can be lowered. If successful, chip circuit density can be increased without any adverse effect on chip performance and reliability.
One method of packaging LSI chips in a module is shown in the September, 1976 (Vol. 19, No. 4) issue of IBM's Technical Disclosure Bulletin, in an article by B. Clark appearing at page 1336. In this arrangement, the semiconductor chip is connected to conductive lands carried by a ceramic substrate via a plurality of solder balls. The lands, in turn, are connected to pins which extend from the module which is enclosed by a metal cap. Heat transfer from the chip conventionally occurs through the solder balls and the lands to the pins and also from the substrate and chip to the metal cover. In this instance, heat transfer is enhanced by the addition of a peripheral flange that forms a fin for additional heat transfer to the ambient air.
In the April, 1977 (Vol. 19, No. 11) issue of IBM's Technical Disclosure Bulletin, in an article by D. Baldereo et al appearing at pages 4165-6, it is suggested that those substrate lands not used for electrical connection be employed as thermal conductive paths connected directly to the module cap. The article by J. Lynch et al in the June, 1977 (Vol. 20, No. 1) issue of IBM's Technical Disclosure Bulletin appearing at page 143, recommended the addition of a conductive piston which was biased into contact with the chip to provide a substantial heat sink mass and the further use of a solid, low-expansion material, such as molybdenum or beryllium oxide, surrounding the piston and biasing means to enhance its thermal conductivity.
An article by E. Berndlmaier et al which appeared at pages 1772-3 of the October, 1977 (Vol. 20, No. 5) issue of IBM's Technical Disclosure Bulletin presented a different approach to the problem. In this arrangement, the chip was mounted in a sealed cavity within the module, which cavity was bellows-like in construction and included a small amount of liquid metal therein. A stud, extending from the module cap, was located in thermal contact within the bellows to provide a large surface area interface with the liquid metal to promote heat conduction to the cap. The use of a thermal medium, such as a liquid metal, thermal fluid or grease, as an aid in promoting heat transfer from the chip is described in the article by A. Arnold et al which appeared in the December, 1977 (Vol. 20, No. 6) issue of IBM's Technical Disclosure Bulletin at pages 2675-6.
The articles by R. N. Spaight and P. Ginnings et al, which appeared respectively in IBM's Technical Disclosure Bulletin in the December, 1977 (Vol. 20, No. 7) issue at page 2614 and the April, 1979 (Vol. 21, No. 11) issue at page 4493, also described the use of a formable thermal transfer medium to enhance heat dissipation from a chip module.
The use of dendritic projections in high temperature applications was noted by J. Cuomo in his IBM Technical Disclosure Bulletin article in the September, 1975 (Vol. 18, No. 4) issue at pages 1239-40. In this article, tungsten dendrites were employed in a heat pipe application as an inert wicking agent. Dendrites have also been suggested for use as a means of providing an increase in surface area for all modes of heat transfer and, specifically, for populating a chip surface to contact a spherically tipped piston in a chip module. This arrangement is described in the article by S. Oktay et al which appeared in IBM's Technical Disclosure Bulletin in the November, 1977 (Vol. 20, No. 6) issue at page 2218. A further use of dendrites to enhance chip cooling is depicted in the February, 1972 (Vol. 14, No. 9) issue of IBM's Technical Disclosure Bulletin at page 2690 in the article by E. Bakelaar et al. In this arrangement, a plurality of solder ball mounted chips have their exterior surfaces lined with dendritic growth as is the lining of the interior surface of a heat pipe which is positioned about the chips. The dendrites on the chip surface and the heat pipe lining are placed into contact with each other and a dielectric fluid which partially fills the heat pipe thereby improving the capillary action of the dielectric coolant and increasing the cooling action on the chips. It has also been taught by Antonucci et al in the December, 1978 (Vol. 21, No. 7) issue of IBM's Technical Disclosure Bulletin at pages 2910-11, that a controlled growth of dendrites can be utilized to increase the tensile strength and decrease the electrical resistance of the solder ball joint between chip and substrate in a module.
None of the foregoing prior art techniques have, however, proven to be entirely satisfactory. The solder ball technique, for example, is inefficient as an aid in removing heat from the chip because the total cross-sectional area of all such bonds is small compared to the chip's surface area. Most of the heat is removed from the chip by natural convention from its top surface.
Limited improvements in cooling efficiency have been obtained through the use of a fluid or solid heat transfer medium, as described above, but such improvements have been gained at the expense of new packaging problems. The employment of a solid piston, for example, introduced a problem of substrate breakage because of thermal expansion mismatches. The utilization of fluid heat transfer mediums caused problems with respect to migration of the medium, interaction with the module components and/or module sealing problems.
It was postulated that a significant improvement in cost/performance chip heat removal could be obtained by utilizing a heat sink of a highly thermally conductive solid material placed in intimate contact with the chip's back surface. Liquid metal appeared to be a satisfactory choice for this role as it was highly conductive and would deform or flow to intimately contact the chip without undue mechanical pressure being required. Further, liquid metal exhibited superior thermal conductivity than other proposed thermal liquids and greases and was chemically stable. However, under the influence of thermal cycling and mechanical vibration it has been found that the liquid metal, or any other thermal fluid, will migrate from the chip-backbond interface with a resultant decrease in cooling efficiency. It has also been found that the liquid metal will tend to form an outer surface oxide film which reduces its thermal conductivity and, therefore, the chip cooling efficiency.